Simulation Study of Some PRMA-based Protocols with ChannelReservation for Data Traffic

X. J. Li and P. H. J. ChongNetwork Technology Research Centre,

Nanyang Technological University,50 Nanyang Drive, Singapore 639798

Email: lixuejun@pmail.ntu.edu.sg, ehjchong@ntu.edu.sg

Abstract —This paper investigates the performance ofpacket reservation multiple access (PRMA) protocols withchannel reservation for data traffic (DR) in a cellularenvironment with fixed channel assignment (FCA).Integrated packet voice and packet data traffic areconsidered in the time division multiple access (TDMA)-based system. Furthermore, three schemes are proposedfor voice-data-integration and compared with the PRMAscheme. Through computer simulation, we show that theperformance of PRMA system with DR is better than thatwithout DR in a cellular environment using FCA, and ourproposed schemes can further improve the performance.

Index Terms — PRMA, PRMA-NDR, MAC, EDGE

I. INTRODUCTION

The rapid development of wireless communicationnetworks entails a strong demand for the efficientutilization of the scarce bandwidth. As we know, wirelessis a broadcast medium, and simultaneous transmission inthe same uplink channel may cause collision, whichimpairs the performance of communication systems.Therefore, medium access control (MAC) schemes arerequired so as to share the limited bandwidth amongpossible user terminals (UTs) in an orderly manner. Manymedium access control protocols have been proposed forfuture packet-based cellular networks. Among them, onefamous multiple access protocol for packet voice isPRMA [1]. It uses slotted-ALOHA for UTs to contend forbandwidth reservation. Upon successful reservation, thetime slot is reserved for the UTs exclusive use. UTs withreservation share the channel in TDMA-like manner.Therefore, PRMA enjoys both the main advantage ofslotted ALOHA (decentralized control over channelaccess) and the main advantage of TDMA (efficientchannel utilization). Furthermore, different types of packettraffics, such as voice, data and video are going to besupported in the future generation wireless networks, e.g.Enhanced Data Rates for Global Evolution (EDGE),which is considered as the 2.75G system [7]. For systemwhere packet-data or packet-voice plays an important role[3], much effort is required in designing multiple accessprotocols. PRMA was originally designed for voice trafficand it does not provide data traffic with channelreservation. However, the effect of channel reservation fordata traffic on the performance of the system in a cellular

environment with fixed channel assignment has not beenstudied.

In this paper, we investigate the performance of PRMAprotocol with data traffic channel reservation (DR) in acellular environment with FCA to support packet voiceand data traffic. With DR, the data subsystemperformance can be improved significantly, especiallyunder heavy voice traffic load condition. Furthermore,three schemes are proposed for voice-data-integration(VDI) traffic based on the PRMA with DR, and comparedwith the PRMA scheme.

This paper is organized into 6 sections. Section IIdescribes the PRMA protocol without/with channelreservation for data traffic. Section III presents theproposed schemes. In section IV, simulation setup andvarious parameters are given. Section V discusses thesimulation results and section VI concludes the paper.

II. PRMA

The PRMA protocol is well-defined in [1] and thedetails of PRMA will not be described in this paper.PRMA was proposed to support packet voice transmissionin a micro-cellular system with negligible propagationdelay from the BS to the user terminals. After that,researchers have proposed to extend PRMA to theapplication to cellular system when the propagation delayis small.

A. PRMA-NDRBy means of statistically multiplexing, PRMA can

achieve higher capacity than TDMA. For example, itperforms well in a single-cell environment. For a systemwith frame size of 20 slots, it has been shown that PRMAcan support 37 simultaneous voice conversations forpacket dropping probability of less than 1% [2]. However,in a cellular environment the performance of PRMA islimited by co-channel interference. In order to reduce thepacket loss probability caused by co-channel interference,channel reassignment (CR) is used. Briefly, CR isachieved by switching an on-going talkspurt on a givenchannel to another channel experiencing less interference.It is described and studied in [5]. For the sake of easycomparison, we denote PRMA without DR as PRMA-NDR.

B. PRMA-DRData traffic channel reservation (DR) is added to the

PRMA protocol, and the resulted scheme is denoted asPRMA-DR. Under PRMA-DR, the channel reservationprocedure of voice user terminals (VTs) is not changed.For data user terminals (DTs), successful transmission ofthe first packet of a message will entitle them a channelreservation. With the aid of the broadcast message fromthe BS, no other UTs will transmit in that channel. Uponfinishing transmission of the last packet of a message, aDT will release its reserved channel by transmitting a nullmessage. Once the BS receives a null message in achannel, it will broadcast that channel is now available. Inorder to give high priority to VTs, the permissionprobability of a DT, pd is set lower than that of a VT, pv.

III. PROPOSED SHEMES

Under low traffic load, PRMA is efficient. However,the performance of the data sub-system can be furtherimproved. In this section, we propose a scheme, PRMA-MAP-DR to purposely improve the performance of datasubsystem without affecting that of the voice sub-systemmuch. Next, PRMA exhibits unstable characteristics underhigh traffic load. Two schemes, PRMA-RFC-DR, andPRMA-UB-DR are proposed to improve the protocolstability without sacrificing the performance of the system.

A. PRMA-MAP-DRThe first proposed scheme is PRMA-DR with modified

adaptive permission probability (PRMA-MAP-DR). Weborrow the idea from [6], where a fuzzy controller isadded to the PRMA scheme to adapt the permissionprobability on a slot-by-slot basis. However, we foundthat it was too troublesome to find the optimalcombination of membership functions and weights ofcontrol rules. In this proposed scheme, we turn to a simplefeedback mechanism to adapt the permission probabilities.First, we sample the voice packet dropping probability, Pd,and the probability of voice packet being interfered with,Pi. Then, we calculate voice traffic index using

id PPvIndex += .

Table 1 shows the algorithm used to adapt thepermission probabilities. One design parameter of PRMA-MAP is the sampling interval. After optimization, it is setto 400 slots. Other parameters of this scheme are thethreshold values LOW_THRES, UP_THRES, the adaptivefactor F, initial values for pv and pd. We temporally setLOW_THRES=0.005, UP_THRES=0.009, F=0.1, then pv

and pd are initialized to 0.3 and 0.1, respectively. All theseparameters have their effect on the performance and wewill investigate the effect of these parameters and find theoptimized set in the future.

Table 1: Pseudo-code for PRMA-MAP Algorithm

B. PRMA-RFC-DRThe second proposed scheme is PRMA-DR with

reservation for contention (PRMA-RFC-DR). Reservationfor contention is done by reserving the next available slotfor the current contending VTs’ retransmission. To givehigh priority to the current contending VTs, newcomer ofVTs and existing contenting DTs are not permitted tocontend. For example, during a frame in slot 2, the BSdetects a collision. Then it checks through the frame andreserves the next available slot, e.g. slot 5. Using the shortfeedback message, the BS informs the collided VTs tocontend slot 5. To other UTs, slot 5 is reserved and theywill not transmit in slot 5. In this way, we attempt toreduce the number of contending VTs during a slot andprompt efficient use of the bandwidth. Note that we alsotry to increase permission probability of VTs if theirprevious attempts for channel reservation fail. However,through simulation we found that this adjustment hasnegligible effect on the performance of the scheme.

C. PRMA-UB-DRThe third proposed scheme is PRMA-DR with uniform

backoff (PRMA-UB-DR). We borrow the idea from thefamous medium access control protocol for wireless localarea network (WLAN)—IEEE 802.11. However, uniformbackoff is used instead of exponential backoff, which isdue to the fact that voice packet can only tolerate limiteddelay. Each VT failed to get a channel reservation willselect a value from the interval (0, VBmax], with equalprobability, as its backoff counter, and increase itspermission probability to pVB. The backoff counter isreduced by 1 after each slot. When the backoff counterreaches zero, it will retransmit if it has permission. Thepermission is again granted by a pseudorandom generator,which is equipped in each user terminal. Otherwise, it willwait until the next available slot and attempt to transmitagain. For a DT failed to transmit a packet, it choosebackoff counter values from a different interval (0, DBmax],with equal probability and set its permission probability topDB. Notice that values of VBmax and DBmax are in unit ofslot. Through intensive computer simulation, we find theoptimal combination of VBmax and DBmax. VBmax is set to16 slots and DBmax is set to 32 slots. Once a VT (or DT)successfully retransmits the current packet, it will reset its

IF vIndex<LOW_THRESpd=(1+F)*pd;

pv=(1-F)*pv;ELSEIF vIndex>UP_ THRES

pd=(1-F)*pd;

pv=(1+F)*pv;ELSE

pd = pd ; pv = pv ; //no changeEND

permission probability to pv (or pd) for next talkspurt (ormessage). The values for pVB and pDB are chosen to be 0.4and 0.2, respectively.

IV. SIMULATION SETUP

A. System ModelIn the simulated system, the geographic area is divided

into 27 hexagonal cells. Each cell has a radius of 600meters. The BS of a cell is equipped with an omni-directional antenna, which means no sectorization isconsidered. The incoming traffic is randomly generated by28124 UTs, which are uniformly distributed with 30meters spacing between any two adjacent UTs. The pathloss model follows that in [4], which is shown below:

XdL += 10log10γ

where L is the path loss in dB; � is the path loss exponent;d is the distance between the UT and the BS; X is a zero-mean Gaussian random variable with a standard deviationof 4dB, which is added to take the shadowing effect intoaccount. The system is considered as interference-limitedas channel noise is ignored. Uniform fixed channelassignment (UFCA) is used and the cluster size is 3. Weconsider a system with 3 different frequency carriers, thenby applying the rule of UFCA, each cell is operating usingone frequency carrier. With a frame size of 20 slots, eachcell has 20 logical channels. Since collisions only occurduring the uplink transmission, only uplink channels areconsidered in the simulated system. During a voice call ora data message transmission, the UT is assumed to bestationary, and the power level is assumed to be constant.Furthermore, upon power up, a UT is always served bythe BS with the highest received power for the UTs pilotsignal.

The duration of each simulation is taken to be14000000 slots (more than 180 minutes). Results from thefirst 2000000 slots (considered as system warm-up period)are ignored in order to avoid the transient effect.Furthermore, results are collected from the central 3 cellsin order to avoid edge effect. For most of the simulationresults, the 95% confidence intervals are within ±10% ofthe average values.

B. Voice Traffic ModelThe voice call is assumed to have a Poisson arrival

process with a mean arrival rate �v calls/second per cell.Each call is of a mean duration of 1/� seconds. Then theoffered voice traffic is �v/� Erlangs. Voice calls consist ofprincipal talkspurts and principal silence gaps, which canbe detected by a slow voice activity detector (SVAD). Thedurations of principal talkspurts and principal silence gapsare assumed to be exponentially distributed with meandurations of t1 seconds and t2 seconds, respectively. Thiscorresponds to a voice activity factor of 0.425.

C. Data Traffic ModelData messages arrive at DTs according to another

Poisson process with mean arrival rate of �d message/secper cell. This process is statistically independent from thatof the voice calls. The length of data messages is assumedto be geometrically distributed. The message is of anaverage length of m packets.

D. System ParametersThe system parameters are shown in Table 2. If the

first packet is received with signal-to-interference-ratio(SIR) greater than SIRsetup, it will get a successfulreservation. For packets received with SIR lower thanSIRmin, they are considered as being interfered with. Notethat data packet being interfered with will be retransmitted,while no retransmission will be done for the voice packetsbeing interfered with. For channel reassignment (CR), thecorresponding parameters prel and Nrel are set to 0.5 and 2,respectively.

Parameters

Values Parameters

Value

Tf (Frame

Time)0.016s m 5

N (Frame

Size)20 pv 0.3

t2 1.35s pd 0.1t1 1s SIRsetup 9dB�v Variable SIRmin 9dB

1/� 120s prel forCR

0.5

�d 5msg/sec/cell

Nrel forCR

2

Table 2: System Parameters

E. Performance MetricsTo measure the system performance, we use the

following metrics: average probability of voice packetdropped (Pd), average probability of voice packet beinginterfered with (Pi), total probability of packet loss (Pt),average access delay per message (Da) and averagetransfer delay per message (Dm). In this paper, weconsider Pt and Dm as grade of service (GOS) for voicesub-system and data sub-system, respectively. Pd isdefined as the ratio of total number of voice packetdropped to total number of voice packets generated; Pi isdefined as the ratio of total number of voice packet beinginterfered with to total number of voice packets generated.Pt is simply equal to the sum of Pd and Pi. Here weconsider an upper limit of Pt of 1%. For a DT, Da refers toaverage time required for a DT to get a channelreservation upon the arrival of a message. Dm refers to theaverage time required for a DT to transmit a message fromitself to its BS.

V. SIMULATION RESULTS

The performance comparison among the 5 schemesabove-mentioned is done through computer simulation.For the sake of comparison, we denote original PRMAwithout DR, PRMA-DR, PRMA-RFC-DR, PRMA-UB-DR and PRMA-MAP-DR as PRMA-NDR, PRMA-DR,RFC-DR, UB-DR and MAP-DR, respectively. In thesimulation, we are considering VDI traffic with5messages/second/cell data traffic. The voice traffic variesfrom 10 Erlangs to 30 Erlangs. We look at how thevarious performance metrics are affected by the increasingamount of voice traffic at a constant level of data traffic.

For voice subsystem, as shown in Fig. 1, the total voicepacket failure probability increases with increasingoffered voice traffic. At Pt = 1%, the values of supportedvoice traffic are about 25.18 Erlangs, 25..86 Erlangs,26.36 Erlangs, 27.09 Erlangs and 27.36 Erlangs forPRMA-NDR, PRMA, MAP, RFC and UB, respectively.Notice that for PRMA-NDR and PRMA, Pt increasessharply beyond 25 Erlangs for the voice traffic, whichcauses instability problem. For our proposed schemes, thetotal voice packet failure probability increases gradually,hence they provide better stability than PRMA-DR.

10 12 14 16 18 20 22 24 26 28 3010

-3

10-2

10-1

100

Offered Voice Traffic (Erlang/Cell)

To

talP

rob

abili

tyo

fPac

ketL

oss

(Pt)

PRMA-NDRPRMA-DRUB-DRMAP-DRRFC-DR

Fig. 1: Pt vs. Voice Traffic Load

As Pt is simply the sum of Pd and Pi, Pt may not reflectthe performance of the voice sub-system in an accurateway. In order to study the performance of the voice sub-system in detail, we look at the Pd and Pi separately. Fig. 2shows the plot of Pd versus voice traffic load. Pd increaseswith increasing among of voice traffic in a exponentialway. Pd is very low at low voice traffic load for all theschemes except UB-DR. Due to uniform backoffmechanism implemented in UB-DR, the probability ofvoice packet dropping cannot be as low as that of otherschemes. PRMA-NDR exhibits instability at high voicetraffic load, Pd increases sharply beyond 25 Erlangs ofvoice traffic. After allowing channel reservation for DTs,the protocol stability is increased. With our proposed twoschemes, the stability is further increased.

10 12 14 16 18 20 22 24 26 28 3010

-7

10-6

10-5

10-4

10-3

10-2

10-1

100

Offered Voice Traffic (Erlang/Cell)

Pro

bab

ility

ofP

acke

tDro

pp

ed(P

d)

PRMA-NDRPRMA-DRUB-DRMAP-DRRFC-DR

Fig. 2: Pd vs. Voice Traffic Load

As shown in Fig. 3, Pi varies with increasing voicetraffic load in a linear way. From 10 Erlangs to 25 Erlangsamount of voice traffic, PRMA-NDR always results inhigher Pi than those schemes with channel reservation fordata user terminals. Beyond 25 Erlangs of voice traffic, Pi

of PRMA-NDR starts to decrease. This is due to the factthat most of the voice packets are dropped, as shown inFig. 2. For the 4 schemes with DT channel reservation,values of Pi are about the same and always increasing.

10 12 14 16 18 20 22 24 26 28 3010

-3

10-2

Offered Voice Traffic (Erlang/Cell)

Pro

bab

ility

ofP

acke

tBei

ng

Inte

rfer

edW

ith(P

i)

PRMA-NDRPRMA-DRUB-DRMAP-DRRFC-DR

Fig. 3: Pi vs. Voice Traffic Load

For data subsystem, after comparing the five schemes,as shown in Fig. 4, we find that PRMA-DR providesmuch lower message transfer delay than PRMA-NDR,especially at heavy voice traffic load condition. MAP-DRgives the lowest message transfer delay; and the other twoproposed schemes also provide lower Dm compared withPRMA-DR. For PRMA-NDR, Dm increases withincreasing amount of voice traffic slowly when the voicetraffic is lower than 25 Erlangs; beyond 25 Erlangs voicetraffic, Dm increases sharply. This is due to the fact thatwithout channel reservation for data user terminals, thecontention level becomes higher and higher withincreasing amount of voice traffic. However, with channelreservation for data user terminals, data message will betransmitted in time and the contention level will not bebuilt up. Therefore, Dm for the four schemes with channelreservation for data user terminals increases gradually.

This indicates that the system capacity is not reached andthese schemes have higher protocol stability.

10 12 14 16 18 20 22 24 26 28 300.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Offered Voice Traffic (Erlang/Cell)

Ave

rag

eM

essa

ge

Tra

nsf

erD

elay

(Dm

)

PRMA-NDRPRMA-DRUB-DRMAP-DRRFC-DR

Fig. 4: Dm vs. Voice Traffic Load

For the average access delay, Da, as shown in Fig. 5,MAP-DR provides the lowest Da and the other twoproposed schemes perform better than PRMA-DR atheavy voice traffic condition. For PRMA-NDR, there isno Da because there is no channel reservation for data userterminals.

10 12 14 16 18 20 22 24 26 28 300

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Offered Voice Traffic (Erlang/Cell)

Ave

rag

eA

cces

sD

elay

(Da)

PRMA-DRUB-DRMAP-DRRFC-DR

Fig. 5: Da vs. Voice Traffic Load

It seems that there is a direct relationship between Dm

and Da for the four schemes with channel reservation fordata user terminals. The lower Da is, the lower thecorresponding Dm is. For a data user terminal during thetransmission of a message, a channel reservation willsecure the successful transmission of the message.Therefore, the faster a DT can get a channel reservation,the faster it can transmit its messages to its BS.

From the simulation results, we can see that afterimplementing DR, the data subsystem performance isimproved significantly. The voice subsystem performanceis also improved at low voice traffic condition. With ourproposed schemes, we are able to not only increase theamount of supported voice traffic at the 1% upper limit ofPt, but also reduce Dm for data traffic.

VI. CONCLUSION

In this paper, we have investigated the performance ofPRMA system with channel reservation for data userterminals in a cellular environment with fixed channelassignment. Simulation results show that channelreservation for data user terminals can reduce contentionlevel and improve the channel utilization. Furthermore, toimprove the performance of the data sub-system withoutaffecting the performance of the voice sub-system much,we proposed one scheme; to improve the stability of theprotocol, we proposed another two schemes. However, thechannel considered in this paper is error-free andtransmission failure is only due to interference. To studythe performance of PRMA in a more accurate way,Rayleigh fading channels should be considered. Moreresults on the effect of Rayleigh fading channels onPRMA will be presented in future work.

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